Virtually all personal computers include a speaker for providing output. The default speaker is typically inexpensive and intended only for basic output with no expectation on the part of the manufacturer or the user that this speaker will have very high fidelity. In fact, virtually all computers sold today use very inexpensive speakers, typically positioned for convenience of the product designer to fit into an already-crowded environment of the computer enclosure with little or no ability to provide a favorable audio environment for the speaker itself. This is dictated in large part by cost concerns, space concerns and resource restraints, since the expected functionality of this speaker is only to provide low-fidelity output.
Such a speaker is typically used to provide audio feedback, for instance, to indicate that the computer has been turned on, or to provide various alert sounds, for example, when the computer needs to attract the user's attention to inform the user that some command has successfully executed, or perhaps that a command cannot execute (printer is out of paper, no printer selected, incorrect page format selected, and many other events).
Particularly with the recent trend to include a CD player in a computer and with increased use of computers for audio playback of CDs or games, there are a large number of extension speakers that can be connected to a computer. Typical speakers can be positioned near or are incorporated into the computer monitor to deliver higher quality sound directly to the user. In general, many of these speakers are designed using many of the techniques well known to speaker designers, such as porting, tuned enclosures, and the like.
Because of the quality difference of these speakers and the standard computer speaker, the standard speaker is typically disabled when an extension speaker is plugged in. This is particularly true when headphones are connected to the speaker since there is an expectation that the user seeks to direct any audio output from the computer solely to the user.
In previous designs, when the user added a set of external speakers to their computer, a simple mechanical switch arrangement shut off the internal speaker(s) of the computer. Referring to FIG. 1, a typical modern computer audio circuit consists of two identical headphone amplifiers A1 11 and A2 12, a summing amplifier A3 18 that converts the stereo signal from A1 and A2 into a single monophonic signal, a loudspeaker amplifier A4 22, a loudspeaker 23, a headphone output jack (J1) 16, and a comparator (CMP1) 19 used to detect the presence of a headphone plug (not shown) at J1 16.
The computer's (10) two audio signals (left 10L and right 10R) are fed to a pair of headphone amplifiers, A1 11 and A2 12, which can typically drive 32.OMEGA. loads. Blocking capacitors C1 24 and C2 25 are used to remove DC voltage that exists on A1 11 and A2 12 from reaching the headphones (not shown), which are plugged into audio jack J1 16. Resistors R1 13 and R2 14 provide a light load to ground to establish a ground potential at J116, thereby defining the voltages on either side of C1 24 and C2 25. R1 13 and R2 14 attenuate "pop" through the headphones whenever a headphone plug is first inserted into J1 16.
The AC-signal from A1 11 (and, correspondingly, A2 12) is voltage limited, and is typically less than 4V peak-to-peak. Due to the blocking action of C1 24 and the pulldown effect of R1 13, the AC signal at the intersection of R1/C1 13/24 will always be less than .+-.2V peak to peak.
Comparator CMP1 19 is connected to the normally-closed contact SW1 17, and has a high-value pullup resistor, R3 15, to Vcc (typically +5V). CMP1's switching threshold is at least 2.5V. R3's value preferably is at least ten times that of R1 13. Therefore, whenever no plug is present at J1 16, the output of CMP1 19, called "Plug.sub.-- Detect" 20, will remain low because its input remains below its switching threshold, regardless of the AC input signal that CMP1 19 receives.
When an audio plug is inserted in J1 16, normally-closed contact SW1 17 is forced to open. Pullup resistor R3 15 then pulls Plug.sub.-- Detect 20 up to Vcc. Plug.sub.-- Detect 20 is now removed from the AC audio signal, and also pulled up to Vcc, so the output 21 of comparator CMP1 19 now switches to a high state. CMP1's output is then used to disable loudspeaker amplifier A4 22 whenever a plug is inserted in the audio output jack, J1 16.
Thus, in the present embodiment of the design, whenever a plug is inserted into the sound output port of a computer, the internal loudspeaker of the computer system is turned off.
Turning to the design of an internal speaker itself, it would be advantageous to provide a speaker with good bass response, and particularly advantageous to provide a sub-woofer for use with a computer system.
Sound engineers understand that reproducing low-frequency sound requires a relatively large enclosure. Design efforts to date have not succeeded in reproducing satisfactory low frequency sound using a small enclosure. Sound engineers continue to seek ways to make a sound reproducing system as compact as possible, yet still provide satisfactory bass response.
Psychoacoustic studies have determined that the human ear is incapable of locating sounds below approximately 200 Hz. That is, an average person can easily hear such sounds, but cannot pinpoint the sound source's location. This means that it is not necessary to have two large speakers to reproduce stereophonic low-frequency sound. In one preferred form, a single "subwoofer" module mixes the stereo signal into a single monophonic signal, while two (or more) smaller "satellite" speakers are used to reproduce mid- to high-frequency stereo sounds. This technique results in a small-appearing sound system consisting of small satellites, while the much larger subwoofer module is conveniently placed out of eye's view. The user's perception is that the tiny satellites "somehow" reproduce very low frequency sound, since the hidden subwoofer is providing the low frequency response.
Many different design techniques are used in the design of a subwoofer enclosure. While some use elaborate baffling in the construction of the sound enclosure, others use a much simpler technique called "porting." In this type of construction, which is incorporated in a preferred embodiment of this invention, an otherwise airtight box has two holes, one for the loudspeaker and another for a "port." A port is simply a tube that is inserted into the port opening. The volume of air contained in the box interacts with the loudspeaker and the air in the port tube. By proper tuning of the port tube length and diameter, low frequency performance of the loudspeaker enclosure is enhanced.
Up until now, all built-in computer sound systems simply mount a loudspeaker to the chassis, without minimal, if any, speaker enclosure, resulting in poor low-frequency sound. Only careful design of a complete loudspeaker system will provide good low-frequency sound reproduction. What is needed is a high quality audio output system for use with a personal computer.